I built in-floor sealed subwoofers consisting of 8 x 8" drivers each (Dayton Audio DCS 205-4). Those were the only 8" drivers which I found to fit in the subfloor cavity with a box built around them. Each sub has the cone surface area of 2 x 15" subs and 1 x 15" sub displacement. The drivers were not the reference type but I had to compromise since they were the only ones which would fit. The whole build was designed around the subfloor cavity restriction.
The purpose of the subs was to use them below 40Hz. The box size gives each driver Qtc=0.9, Vb=5 Ltr, , Fb=82Hz .
When the house was still renovated and did not have the drywall installed yet I tested them both with 120Hz x-over at full displacement and they played really loud - I would say around 120dB at 120Hz. The garage floor concrete slab was shaking .
Each subwoofer has 2 boxes - the first one is the "drivers head", the second box is the "cavity box". Assembled total length is 10.5ft, height 9.25", width 14".
The subwoofer boxes are bolted together and have a removable cover for bolts access. They were installed in the basement ceiling - between the floor joists. The boxes are separated from the floor structure and don't transfer vibrations to the house. The drivers are mounted opposed to each other to cancel any vibrations too. The boxes are suspended by multiple metal straps (like a swing cradle). For safety there are aircraft cables spun across the bottom too (not tensioned, slightly loose and taped to the boxes to prevent rattling).
The subwoofer ports are covered with the same covers used for the heating ducts in the house and you can't tell that they are there.
When there was no furniture in the house yet you could feel the floor bending from the air pressure whenever the subs played.
Now when I have them set up below 40Hz they are enough for movie playing. For this open floor plan and the music levels I enjoy occasionally though I would need 4x that displacement. That's why there is another project on the go for few years already - but that's another topic altogether .

Here are the in-floor subwoofer design drawings..
I'm using Crown XLS 2500 to power both subs. You can use Crown XLS 1500 too. Plenty of power to drive them both to Xmax.
The combined cone surface per sub equals 21" sub. The excursion is 8mm only though. Displacement equals 15" sub each subwoofer box.

Brilliant concept and fantastic execution! I’m a big proponent of putting tactile transducers in seating risers, so when I saw the title, I had to know the whole story. I’m curious- you seem to have gone to a lot of trouble to decouple these from the floor; did you consider letting them do some “bass shaking”? Or is that a natural side effect even with all the decoupling?

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